The IZ4 Spreading Code Family Incorporated into the NavIC L1 SPS Signal

To meet the nation's positioning, navigation, and timing needs, ISRO has developed a regional navigation satellite system called NavIC (Navigation with Indian Constellation).

Global Navigation Satellite Systems (GNSS) use periodic sequences with low correlatio called spreading codes for satellite acquisition and ranging, essential for accurate positioning, time, and velocity. The required length of these codes is based on the onboard atomic clock frequency of 10.23 MHz. In the L1 frequency band, GPS and BeiDou (BDS) use spreading codes of length 10230, derived by padding or truncating Weil sequences of prime-period 10223 or 10243. However, such modifications can degrade correlation performance.

At IISc, in collaboration with ISRO scientists Dileep Dharmappa (ISTRAC) and Sugandh Mishra (SAC), Professor Kumar devised a novel approach: interleaving five Z4-linear sequences of period 2046 to create a 10230-length sequence. This Interleaved linear (IZ4) family matches or surpasses the balance and even correlation performance of GPS and BDS codes. Notably, it improves maximum even cross-correlation by 4.4 dB and offers comparable odd-correlation performance. The sequences are also efficiently generated via shift-register-based implementations.

A joint IISc-ISRO Indian patent has been granted. ISRO has adopted the IZ4 spreading codes in NavIC's new SPS (civilian) L1 signal, with the first satellite using it, NVS-01, launched on May 29, 2023.

Acknowledgement:

This work was made possible through the strong encouragement and support of senior administrators and faculty at IISc, as well as senior administrators and scientists at ISRO.

References

1. ISRO webpage summarizing the high-level IISc-ISRO meeting held on June 17, 2021.
   (Link to ISRO website)
2. NavIC Signal-in-Space Interface Control Document for Standard Positioning Service in L1 Frequency, Indian Space Research Organisation, August 2023.
   (Link to ICD)
3. P. V. Kumar, D. Dharmappa, and S. Mishra, "Method and System for Generating Spreading Codes Based on Interleaved Z4-Linear Sequences for Navigation Systems," Indian Patent No. 383332, granted to IISc and ISRO, November 2021.
   (Patent Certificate)
4. P. V. Kumar et al., "Interleaved Z4-Linear Sequences with Low Correlation for Global Navigation Satellite Systems", IEEE Transactions on Information Theory, March 2024.
   (Link to paper)